I've often wondered where this advice came from. I can't see that it
would make any difference - once the clematis starts growing it creates
its own shade anyway.

In the "Illustrated History of Gardening" (1884 - 1888) there is no
mention of a need to cover the roots. But the answer may be in
"Cassell's Dictionary of Gardening" (1913). Here it notes "The Clematis
is subject in some gardens to a disease which causes it to die off.
This appears to be caused by an unknown constituent in the soil in these
gardens, but in some where the clematis has previously failed it has
been found that then plants will grow satisfactorily if the lower
portions of the stems are shaded from the sun by other plants. This is
worthy of a trial in gardens where the plant has hitherto been lost".

So to get round clematis wilt (assuming that is what the article is
referring to), one suggestion was to shade the lower part of the plant.
We now, of course, use "maximum shading" by burying the plant much
more deeply than normal!

If that is the explanation for generally shading the roots, it is
interesting how it has become corrupted and expanded to cover (no pun
intended) clematis planting generally.

--

Jeff